Conventional apparatus for molding plastics comprise an upper mold and a lower mold which include opposed blocks each having pairs of opposed cavities in each of the fitting surfaces of the molds. An auxiliary runner is formed between the opposed cavities and communicates with the cavities through gates. The center block of the apparatus has in its center a cull portion from which main runners branch out to communicate with the auxiliary runners.
When semiconductor chips are to be enclosed with plastics by the molding apparatus, lead frames carrying semiconductor chips are set in place between the fitting surfaces of the upper and lower molds, and the molds are thereafter clamped together. A preheated molding material, such as epoxy resin or like thermosetting synthetic resin, is fed to a pot in the center of the upper mold and plasticized in the pot by being subjected to pressure with a plunger. The plasticized material is injected into the cavities through the main runners, auxiliary runners and gates and is molded with the semiconductor chips enclosed with portions of the material.
When the resin material is thus molded with the apparatus described above, portions of the material are also molded in the cull portion, main runners, auxiliary runners and gates, so that the operation affords a seriously reduced yield of about 65% if highest, or as low as 20% or lower in extreme cases. Since the molding material is thermosetting and is therefore not resuable, the waste of the material results in a great economical loss.